Pipe Sizing → Water Heater Sizing → Heat Loss →
Imperial (Btu/h / ft)
Metric (kW / m)
Gas
Natural gas
Propane (undiluted)
Allowable Pressure Drop
0.5 in w.c. (125 Pa) — standard, 7 in w.c. supply
Pipe Material
Schedule 40 steel
Copper tubing (K & L)
Polyethylene (PE) — outdoor underground only
Total Connected Load (Btu/h)
Longest Length — Meter to Farthest Appliance (ft)
Heating Value (Btu/ft³)
Use your gas utility's heating value if you have it (natural gas is typically 1,000–1,050 Btu/ft³).
Calculate
Reset
Copy Result
Minimum Pipe Size
—
Enter values above and click calculate.
How It Works
This uses the longest-length method from NFPA 54 / the
IFGC (and CSA B149.1): size the pipe for the total connected load of
every appliance it feeds, using the length from the meter (or
second-stage regulator) to the farthest appliance.
Q = (D × 19.17 × (H ÷ (Cr × L))0.206 )1/0.381
This is IFGC Equation 4-1 (low pressure) solved for flow: Q in ft³/h,
D the pipe's inside diameter in inches, H the pressure drop in in w.c.,
L the length in feet, and Cr = 0.6094 for natural gas or 1.2462 for
propane. Load in Btu/h is converted to ft³/h with the heating value.
Built to stay on the safe side
The equation reads up to 2% above the code's printed tables, so every
capacity here is reduced by 2%. Results never exceed the printed IFGC
tables they were checked against. Runs under 10 ft are sized at 10 ft.
Materials
Schedule 40 steel, copper tubing (Type K and L; some utilities restrict
copper where gas contains sulfur compounds), and PE for outdoor
underground use only. CSST is sized from the
manufacturer's EHD capacity table for that product, so it isn't included.
CSA B149.1 (Canada)
B149.1 uses the same longest-length approach, but its low-pressure tables
build in a 1.2 fittings factor, so they allow roughly 9% less capacity
than the IFGC tables. A B149.1 mode is coming once it can be checked
against published Canadian tables. Until then, verify Canadian
installations against B149.1 Annex A.